Transmission system implementing automated directional shift braking

ABSTRACT

A transmission system is disclosed for use with a machine. The transmission system may have a first clutch to transfer power in a first direction, a second clutch to transfer power in a second direction, a brake, a sensor to generate a first signal indicative of speed, and an input device to generate a second signal indicative of a desire to shift directions. The transmission system may also have a controller to cause disengagement of the first directional clutch in response to the second signal, and to cause the brake to apply retarding torque. The controller may also be configured to determine an error value based on the second signal and a target transmission system speed, to selectively increase retarding torque when the error value increases, and to selectively transfer retarding torque to the second directional clutch when a value of the second signal is less than a threshold value.

TECHNICAL FIELD

The present disclosure relates generally to a transmission system and,more particularly, to a transmission system implementing automateddirectional shift braking.

BACKGROUND

A mobile machine, such as a wheel loader, a track-type-tractor, a motorgrader, or a haul truck, includes a transmission that transfersrotational power from an engine of the machine to wheels or othertraction devices. A typical transmission includes two directionalclutches (e.g., a forward clutch and a reverse clutch) that areselectively engaged to alter a power flow path between the engine andthe wheels. During forward travel, only the forward clutch should beengaged to transfer mechanical rotation from the engine through thewheels in a forward direction. During reverse travel, only the reverseclutch should be engaged to transfer the same mechanical rotationthrough the wheels in a reverse direction. During a directional shiftchange (i.e., when shifting between the forward and reverse directions),a first of the two directional clutches will release at about the sametime as or before the second of the directional clutches engages.

If the directional shift change is attempted while the machine ismoving, significant momentum aligned with the first travel directionmust first be dissipated before the machine can begin accelerating inthe second and opposite travel direction. This momentum is generallydissipated via friction material inside the engaging clutch. That is,the second clutch will generally slip until sufficient power has beendissipated to avoid shock-loading and damage to the remaining drivetraincomponents of the machine. During slipping, some of the frictionmaterial of the second clutch is worn away. In some situations, anoperator of the machine may attempt to manually brake the machine duringshifting between travel directions to reduce the amount of momentum thatmust be absorbed by the friction material of the second clutch. By doingso, the lives of the clutches may be prolonged and/or the directionalshift may be completed more quickly and/or at higher speeds. It can bedifficult, however, for the operator to use the right amount of brakingand to properly time engagement/disengagement of the clutches to allowfor smooth and efficient shifting without causing undue wear or damageof the drivetrain components.

An exemplary transmission system is disclosed in U.S. Pat. No. 8,880,303of Ishikawa et al. that issued on Nov. 4, 2014 (“the '303 patent”). Thetransmission system of the '303 patent includes a vehicle control unitthat detects a vehicle travel speed and prevents a shift change in adirection opposite to a vehicle traveling direction, as long as thevehicle travel speed is faster than a first speed. Specifically, whenthe vehicle control unit detects that a user has selected a shift rangein a direction opposite to the vehicle traveling direction, while thevehicle travel speed is faster than the first speed, the vehicle controlunit carries out a compulsory deceleration via hydraulic brakes to stopthe vehicle instead of permitting the shift change. The shift change isonly then permitted after the vehicle is stopped and the engine of thevehicle is in an idle state. Since the vehicle control unit ignores anerroneous shift change in a direction opposite to the vehicle travelingdirection during high speed travel, a breakdown of the vehicle isprevented.

Although the strategy employed by the vehicle control unit of the '303patent may have some effect on vehicle component life, the focus of thestrategy is accommodating operator error. In particular, the vehiclecontrol unit may do little to improve shift quality during a desiredhigh-speed shift. In addition, the vehicle control unit may not beapplicable to situations where the operator has not made an error inrequesting a directional shift change.

The disclosed transmission system is directed to overcoming one or moreof the problems set forth above and/or other problems of the prior art.

SUMMARY

In one aspect, the present disclosure is directed to a transmissionsystem for use with a machine having an engine and a traction device.The transmission system may include a first clutch configured totransfer power from the engine to the traction device in a firstdirection, a second clutch configured to transfer power from the engineto the traction device in a second direction opposite the firstdirection, a brake configured to apply a retarding torque to thetraction device, a sensor configured to generate a first signalindicative of a speed of the transmission system, and an input deviceconfigured to generate a first signal indicative of an operator's desireto shift power transfer directions from the engine to the traction. Thetransmission system may also include a controller in communication withthe first clutch, the second clutch, the brake, the sensor, and theinput device. The controller may be configured to cause disengagement ofthe first clutch in response to the second signal, and to cause thebrake to apply the retarding torque to the traction device, and todetermine. The controller may also be configured to determine an errorvalue as a function of the second signal and a target transmissionsystem speed, to selectively increase the retarding torque applied bythe brake when the error value increases, and to selectively transferthe retarding torque from the brake to the second directional clutchwhen a value of the second signal is less than a threshold value.

In another aspect, the present disclosure is directed to a method ofautomatically shifting traveling directions of a machine having anengine and a traction device. The method may include transferring powerfrom the engine through a first directional clutch to the tractiondevice in a first direction, and receiving input indicative of anoperator's desire to transfer power from the engine to the tractiondevice in a second direction. The method may further include disengagingthe first directional clutch in response to the input, and causing abrake to apply a retarding torque to the traction device. The method mayfurther include sensing a speed of the transmission, determining anerror value as a function of the speed and a target transmission speed,selectively increasing the retarding torque applied by the brake whenthe error value increases, and selectively transferring the retardingtorque from the brake to a second directional clutch of the transmissionwhen the speed is less than a threshold value.

In another aspect, the present disclosure is directed to a machine. Themachine may include an engine, a traction device, and a transmissionhaving a first directional clutch, a second directional clutch, and aplurality of gear ratio clutches. The transmission may be configured totransfer power from the engine to the traction device in first andsecond directions and throughout a range of speed-to-torque ratios. Themachine may also include a brake configured to apply a retarding torqueto the traction device, an input device configured to generate a signalindicative of an operator's desire to shift power transfer directions ofthe transmission throughout operation of the transmission within therange of speed-to-torque ratios, a sensor configured to generate asecond signal indicative of a speed of the transmission, and acontroller in communication with the transmission, the brake, the inputdevice, and the sensor. The controller may be configured to causedisengagement of the first directional clutch in response to the secondsignal, and to cause the brake to apply the retarding torque to thetraction device. The controller may also be configured to determine anerror value as a function of the second signal and a target transmissiondeceleration rate, to selectively increase the retarding torque appliedby the brake when the error value increases, and to selectively transferthe retarding torque from the brake to the second directional clutchwhen a value of the second signal is less than a threshold value.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an isometric illustration of an exemplary disclosed machine;

FIG. 2 is an diagrammatic illustration of an exemplary disclosedtransmission system that may be used with the machine of FIG. 1;

FIG. 3 is a flowchart depicting an exemplary disclosed control algorithmassociated with operation of the transmission system of FIG. 2; and

FIG. 4 includes graphs illustrating performance parameters of thetransmission system of FIG. 2 during implementation of the controlalgorithm of FIG. 3.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary mobile machine 10. In the depictedembodiment, the machine 10 is a wheel loader. It is contemplated,however, that machine 10 may embody another type of mobile machine suchas a track-type-tractor, a motor grader, an articulated haul truck, anoff-highway mining truck, or another machine known in the art. Themachine 10 may include an operator station 12, one or more tractiondevices 14 that support the operation station 12, and an engine 16operatively connected to drive the traction devices 14 in response toinput received via the operator station 12.

The operator station 12 may include interface devices that receive inputfrom a machine operator indicative of desired machine maneuvering.Specifically, the operator station 12 may include one or more interfacedevices 18 located proximate a seat 20 for use by a machine operator.The interface devices 18 may initiate movement of machine 10 byproducing displacement signals that are indicative of desired machinemaneuvering. In one embodiment, the operator interface devices 18include a shift button. As an operator manipulates the shift button, themachine 10 may be caused to automatically shift travel directions (e.g.,from forward to reverse or from reverse to forward). It is contemplatedthat an operator interface device 18 other than a shift button mayadditionally or alternatively be provided within the operator station 12for movement control of machine 10, if desired.

As shown in FIG. 2, the engine 16 may be operatively connected to thetraction devices 14 by way of a transmission system 22. The transmissionsystem 22 may include, among other things, a torque converter 24, and atransmission 26 that is operatively connected to the engine 16 by way ofthe torque converter 24. The transmission 26 may, in turn, be connectedto the traction devices 14 directly or indirectly (e.g., by way of afinal drive—not shown), as desired. In the depicted example, the engine16 is an internal combustion engine (e.g., a diesel, gasoline, ornatural gas engine).

The torque converter 24 may be a conventional type of torque converterhaving an impeller 28 connected to an output of the engine 16, and aturbine 30 connected to an input of transmission 26. In someembodiments, the torque converter 24 may additionally have a lockupclutch (not shown) disposed between the engine output and thetransmission input in parallel with the impeller 28 and the turbine 30.In this configuration, as the engine output rotates the impeller 28, aflow of fluid may be generated and passed through the turbine 30,causing the turbine 30 to rotate and drive the transmission input. Thisfluid connection, while capable of passing power from the engine 16 tothe transmission 26, may drive the transmission input to rotate at adifferent speed and/or with a different torque than what is receivedfrom the engine output. This may allow a load of the transmission 26 tobe somewhat isolated from the engine 16, such that shock-loading of theengine 16 does not occur. In addition, the fluid coupling facilitated bythe torque converter 24 may allow for torque multiplication in someapplications. It is contemplated that the torque converter 24 could beomitted, if desired, and the transmission 26 connected directly to theoutput of the engine 16, if desired.

The transmission 26 may be a step-change transmission having multipledistinct gear ranges in both a forward travel direction and in a reversetravel direction. It should be noted that other types of transmissions(e.g., a hydraulic or hybrid transmission) may be used, if desired. As astep-change transmission, the transmission 26 may include a front box 32housing a forward travel clutch 34 and a reverse travel clutch 36, and arear box 38 housing a plurality of the different speed range clutches40. The clutches 34 and 36 may be selectively engaged to connect amechanical rotation of the turbine 30 to an input shaft 42 of the rearbox 38. When the forward travel clutch 34 is engaged, the tractiondevices 14 may be caused to rotate in a forward travel direction. Whenthe reverse travel clutch 36 is engaged, the traction devices 14 may becaused to rotate in a reverse travel direction. The speed range clutches40 may be selectively engaged and disengaged based on any number offactors known in the art to adjust a speed-to-torque ratio of thetransmission 26. For example, the speed range clutches 40 may be engagedand disengaged based on a travel direction, based on a travel speed,based on loading, when commanded by an operator, etc. It is contemplatedthat the clutches 34, 36, and 40 could alternatively be housed withinthe same box, if desired.

The clutches 34, 36, and 40 may all be hydraulic-type clutches. Inparticular, the clutches 34, 36, and 40 may each be configured toselectively receive a flow of pressurized fluid that causes engagementof portions of a gear train (not shown) within the transmission 26. Eachof the clutches 34, 36, and 40 may include an interior actuating chamber(not shown) that, when filled with pressurized fluid, displaces a piston(not shown) toward one or more input disks (not shown) and one or moreoutput plates (not shown) that are interleafed with the input disks. Thecombination of input disks and output plates are known as a clutch pack.As the actuating chamber is filled with fluid, the piston “touches up”to the clutch pack to press the input disks against the output plates.This may initiate engagement of the clutch, and power may begin to betransferred between the input disks and the output plates throughfriction. As the pressure of the fluid inside the actuating chamberincreases, a strength of the engagement and the associated powertransfer likewise increases. That is, when the pressure of the fluid islow, slippage between the input disks and the output plates may occurand the associated power transfer efficiency loss through the clutch maybe absorbed by friction material of the input disks and/or outputplates. When the pressure of the fluid is high, the clutch may no longerslip and a majority of the power transferred into the clutch via theinput disks may pass back out of the clutch to downstream components viathe output plates. The combination of engaged clutches may determine theoutput rotational direction and the speed-to-torque ratio of thetransmission 26.

A timing between clutch engagement/disengagement can affect shiftquality. For example, if an engaging one of the clutches 34, 36 isfilled with pressurized fluid while the machine 10 is still moving in afirst direction, the engaging one of the clutches 34, 36 must firstabsorb the momentum associated with the machine's movement in the firstdirection before acceleration of the machine 10 in the second directioncan begin. This momentum absorption may occur within the frictionmaterial of the clutch pack. And if the engaging one of the clutches 34,36 fully engages too quickly (i.e., before enough of the momentum isabsorbed), the engagement can result in shock-loading that can damagemachine components. For this reason, engagement/disengagement of theclutches 34 and 36 may be selectively coordinated with engagement of abrake 43.

In the disclosed embodiment, the brake 43 is a hydraulic wheel brakeassociated with the traction device 14. That is, the brake 43 may beprovided with a pressure that functions to generate friction on materialinside the brake used to slow the rotation of the traction device 14. Insome instances, the force is the result of a hydraulic pressure beingapplied to the friction material. In other instances, the force is theresult of spring force being applied as hydraulic pressure is reduced.Other configurations may also exist, and the brake 43 may take any formknown in the art. For example, the brake 43 may be an external drybrake, an internal wet brake, or another type of brake. For the purposesof this disclosure, the brake 43 may be considered a part of thetransmission system 22.

A controller 44 may be in communication with the interface device 18,clutches 34 and 36, and brake 43, and configured to automatically adjustoperation of the transmission system 22 during a directional shift ofthe machine 10. For example, based on an operator's desire to shifttravel directions, the controller 44 may selectively cause disengagementof one or clutches 34 and 36, activation of brake 43, and engagement ofthe other of clutches 34 and 36. This automated control may result insmooth directional changes with little component wear and shock-loading.In addition, the automated control may allow for higher-speeddirectional changes.

The controller 44 may include a memory, a secondary storage device, aclock, and one or more processors that cooperate to accomplish a taskconsistent with the present disclosure. Numerous commercially availablemicroprocessors can be configured to perform the functions of thecontroller 44. It should be appreciated that the controller 44 couldreadily embody a general transmission or machine controller capable ofcontrolling numerous other functions of the machine 10. Various knowncircuits may be associated with the controller 44 includingsignal-conditioning circuitry, communication circuitry, and otherappropriate circuitry. It should also be appreciated that the controller44 may include one or more of an application-specific integrated circuit(ASIC), a field-programmable gate array (FPGA), a computer system, and alogic circuit configured to allow the controller 44 to function inaccordance with the present disclosure.

In some embodiments, the controller 44 may rely on sensory informationwhen regulating the operations of the transmission system 22. Forexample, in addition to receiving the signals generated by interfacedevice 18 requesting the automated directional shift change, thecontroller 44 may also communicate with one or more sensors to detectactual pressures inside the transmission system 22 that are indicativeof a shift status. These sensors could include, for example, atransmission output speed sensor 46, one or more clutch pressure sensors48, and a brake sensor 50. The controller 44 may implement the automaticadjustments of the transmission system 22 described above based on thesignals generated by these sensors.

FIG. 3 is a flowchart depicting an exemplary method of transmissioncontrol that may be implemented by the controller 44, while FIG. 4includes a compilation of graphs illustrating performance parameters ofthe transmission system 22 during an exemplary automated control event(i.e., during an automated directional shift). These figures will bediscussed in the following section to further illustrate the disclosedconcepts.

INDUSTRIAL APPLICABILITY

While the transmission system of the present disclosure has potentialapplication in any machine requiring multiple speed and torquetransmission levels in two directions, the disclosed transmission systemmay be particularly applicable to wheel loaders, motor graders,track-type-tractors, articulated haul trucks, off-highway mining trucks,and other heavy construction machines. Such machines have specificgearing and cycle time requirements that the disclosed transmissionsystem may be especially capable of meeting. The disclosed transmissionsystem may improve the shift quality of any machine by selectively andautomatically affecting transmission characteristics such as clutchpressures, clutch engagement timings, and brake pressures. Operation ofthe transmission system 22 will now be described in detail.

The shifting of transmission 26 may be done at any time that the machine10 is operational. Shifting may be initiated manually or automatically,and may include directional shift changes and/or gear ratio shiftchanges. To initiate an automated directional shift change, the operatorof the machine 10 may depress or otherwise manipulate the interfacedevice 18 (e.g., the shift button—referring to FIG. 2). The operator maydepress interface device 18 when moving in any direction, in any gear,and at any speed. The controller 44 may continuously monitor the use ofthe interface device 18 to determine if an automated shift in traveldirection is desired (Block 300). The controller 44 may consider theautomated directional shift to be desired by the operator when theinterface device 18 has been depressed and generates a correspondingsignal. Control may loop through block 300 until the signal isgenerated. In the example depicted in FIG. 4, receipt of the shiftbutton signal may correspond with a time T₀.

At some point after receiving the signal from the interface device 18,the controller 44 may cause the appropriate one of the clutches 34 and36 to disengage (Block 305). For example, when machine 10 is travelingin the forward direction and the operator depresses the interface device18, the controller 44 may disengage forward the clutch 34 such thatengine 16 is no longer actively driving the traction devices 14 in aforward direction. And when machine 10 is traveling in the reversedirection and the operator depresses interface device 18, the controller44 may disengage reverse the clutch 36 such that the engine 16 is nolonger actively driving the traction devices 14 in a reverse direction.In the flowchart of FIG. 3 and in the example of FIG. 4, the one of theclutches 34 and 36 being disengaged during a directional shift may beconsidered the “offgoing clutch”, whose operation is represented by apressure curve 400 in FIG. 4. And the other of the clutches 34 and 36may be considered the “oncoming clutch”, whose operation is representedby a pressure curve 410. As can be seen in FIG. 4, the disengagement ofthe offgoing clutch may not be instantaneous. That is, the controller 44may initiate clutch disengagement by causing a pressure reduction in theclutch pack of the offgoing clutch, which may take place over a periodof time related to a specific configuration of the offgoing clutch.During this disengagement, the amount of power being transmitted throughthe offgoing clutch may reduce gradually in a linear or non-linearmanner. As can be seen in the curve 410, pressurization of the oncomingclutch may not begin during disengagement of the offgoing clutch. Infact, a significant time lag may even exist between disengagement of theoffgoing clutch and engagement of the ongoing clutch, in some instances.It may even be possible, in some situations, for the ongoing clutch tobegin pressurizing before the offgoing clutch is commanded to reduce itspressure.

Throughout the process of automated directional shifting, the controller44 may continuously monitor the output speed of the transmission 26(i.e., the TOS) and, after disengagement of the offgoing clutch, thecontroller 44 may compare the TOS to a threshold speed value TOS₁ (Block310). The TOS may be monitored by way of speed sensor 46 and may berepresented in the example of FIG. 4 by a speed curve 415. The thresholdvalue TOS₁ may be a value above which directional shifting withoutbraking may be problematic. For example, when the TOS is greater thanthe threshold speed TOS₁, shifting directions without braking could bedamaging to components of the machine 10, could reduce a life of thecomponents, and/or could result in unstable (e.g., jerky oruncomfortable) machine operation. The threshold speed TOS₁ may bedifferent for each machine 10, each transmission 26, and/or each clutchconfiguration. The threshold speed TOS₁ may also be configurable by theoperator of (or another entity associated with) machine 10. In theexample of FIG. 4, the TOS at time T₀ is shown as being greater than thethreshold speed TOS₁.

When the controller 44 determines at block 310 that the TOS is notgreater than the threshold speed TOS₁ (i.e., when the TOS is equal to orless than the threshold speed TOS₁), the controller 44 may modulate thepressure in the oncoming clutch to complete the directional shiftwithout activating the brake 43 (Block 315). That is, the controller 44may cause the pressure inside of the clutch pack of the oncoming clutchto increase until full engagement is achieved. The pressure may bemonitored via the clutch pressure sensor 48, and the increase inpressure may take place over a period of time related to theconfiguration of the oncoming clutch. The increase may be linear ornon-linear, as desired. Control may return from block 315 to block 300.

When the controller 44 determines at block 310 that the TOS is greaterthan the threshold speed TOS₁, the controller 44 may initiateapplication of brake 43 at a substantially constant rate of increase(Block 320). For example, the controller 44 may generate a command thatcauses the pressure on the friction material of brake 43 to increase ata substantially constant rate. For the purposes of this disclosure, theterm “substantially constant” may be defined as “constant withinengineering tolerances.” In the example of FIG. 4, operation of brake 43at any given point in time may be represented by a command the curve 420as a percent of a maximum brake command, with the application of brake43 initiating after a time T₁. It should be noted that, although theapplication of brake 43 is shown as initiating after the offgoing clutchpressure has reduced to about zero, it is contemplated that brake 43could be applied at some point during the depressurization of theoffgoing clutch (i.e., before the pressure has reached zero), ifdesired. For the purposes of this disclosure, the term “about” may bedefined as “within engineering tolerances.” The constant rate of brakingincrease may be represented by the slope of curve 420 after time T₁, andmay be set at any desired rate. In the disclosed example, the constantrate of braking increase may correspond with a maximum braking that isacceptable in terms of operator feel. For example, the constant rate ofbraking increase may be about the same as a recent maximum braking ratethat was manually implemented by the operator. This increase in thebraking rate may function to rapidly pre-calibrate the controller 44 forsubsequent braking changes. The pressure on the friction material of thebrake 43, and therefore the actual braking of the machine 10, may bemonitored via the sensor 50.

It is contemplated that the constant increase in the command of brake 43during completion of block 320 may not necessarily start at a point ofzero braking, as shown in FIG. 4. In particular, it may be possible forthe command of the brake 43 to first step from zero to a minimum levelbefore the command is caused to increase at the constant rate, ifdesired. The step increase may be associated only with the filling ofchambers/pistons/passages within brake 43 (i.e., associated only withthe preparation of brake 43 to begin retarding machine motion), withoutsignificant retarding actually being realized. It is also contemplated,however, that the command of brake 43 implemented during completion ofblock 320 could alternatively step from zero to a level above theminimum level required for mere brake preparation, in some situations.For example, the command could step from zero to a preliminary level ofbraking that actually retards machine 10 somewhat, but that is alsoknown to be less than required to achieve the target deceleration rate.Both of these optional steps may help reduce a time required fordirectional shifting. One or both of these steps may be selectivelyimplemented based on any number of different condition, for examplebased on a starting gear when directional shifting is required, a pitchof the machine 10, TOS at a time of shift request, estimated machineweight, etc.

At some point during the application of brake 43, the machine 10 willbegin to decelerate, and the deceleration may be detectable via the peedsensor 46. Accordingly, the controller 44 may take a time-basedderivative of the signal from the speed sensor 46, filter the derivativeto remove noise from the signal, and compare the filtered derivative toa threshold deceleration (Block 325). The primary purpose of block 325may be to determine at what brake command a noticeable deceleration ofmachine 10 occurs. Noticeable deceleration may be considered to haveoccurred only when the filtered derivative of the TOS is greater thanthe threshold deceleration, and the threshold deceleration may bedifferent for each machine. Control may cycle from block 325 throughblock 320 until the filtered derivative of TOS becomes greater than thethreshold deceleration. That is, the controller 44 may continue toincrease the braking command of brake 43 at the constant rate until themachine 10 starts to decelerate noticeably. In the example of FIG. 4,the machine 10 starts to decelerate noticeably at a time T₂.

Once the machine 10 starts to noticeably decelerate (i.e., once thefiltered derivative of the TOS becomes greater than the thresholddeceleration), the controller 44 may begin to apply brake 43 at avariable rate such that the deceleration rate of TOS is maintained aboutthe same as a target deceleration rate (Block 330), which is representedin FIG. 4 as a line 440. In particular, the controller 44 maycontinuously determine a slope of the curve 415, and selectively adjustthe rate of brake application (i.e., the brake command and rate ofchange of the brake 43) such that the slope of curve 415 after time T₂remains about the same as the slope of line 440. By comparing only theslope of curve 415 to the slope of line 440 (rather than continuouslycomparing a filtered time-based derivative to a desired decelerationrate), the amount of calculations being performed by the controller 44may be lower. This calculation reduction may result in a more responsivesystem and/or require less computing power. The target deceleration rateused in block 330 may be greater than the threshold deceleration used inblock 325. In one embodiment, the threshold deceleration is some percentof the target deceleration that allows the actual deceleration ofmachine 10 to approach the target deceleration at time T₂, withoutovershooting the target deceleration.

When controlling braking of machine 10 at the variable rate (i.e., whenimplementing block 330), the controller 44 may utilize aproportional-integral (PI) algorithm. The PI algorithm utilized by thecontroller 44 may be a control-loop algorithm that calculates an errorvalue as a difference between the slope of curve 415 and the slope ofcurve 440 (regardless of offset), and generates a braking command changethat attempts to reduce the error. It should be noted that, during thevariable rate control portion of the automated shift, a maximum limit onbraking increase may still be imposed. In the disclosed embodiment, thislimit may be the same rate of braking increase used between times T₁-T₂,and corresponds with a maximum amount of braking normally acceptable tothe operator (i.e., a comfortable braking rate).

The controller 44 may determine the braking command at block 330 in aunique way. Specifically, since the goal of the controller 44 is not toforce the TOS to match a specific target TOS value, but instead to bendthe TOS parallel to the TOS target line, the integral error may onlyaccrue when the TOS diverges from the target TOS line. That is, the TOSmay be offset from the target TOS line without incrementing the integralerror, as long as the TOS is not diverging from the target TOS line.

During application of brake 43 at the variable rate, the controller 44may continuously check to see if the oncoming clutch is capable ofassuming the braking torque currently being carried by brake 43 (Block335). That is, at time T₂, the torque being applied by brake 43 todecelerate machine 10 may be too large for proper dissipation by thefriction material of the oncoming clutch. In other words, if brake 43were to stop retarding the motion of machine 10 and the momentum ofmachine 10 instead began passing through the friction material of theoncoming clutch, the oncoming clutch could wear prematurely or evenfail. At some point after time T₂, however, the momentum of the machine10 will have reduced to a level that can be properly dissipated by thefriction material of the oncoming clutch. This level is represented by atransmission output speed TOS₂ in the example of FIG. 4. Thus, at block335, the controller 44 may compare the current TOS to TOS₂ to determineif the oncoming clutch is capable of assuming the retarding torque frombrake 43. As long as the current TOS is greater than TOS₂, control maycycle from block 335 back to block 330.

When the comparison of block 335 indicates that the current TOS is equalto TOS₂, the controller 44 may freeze the current braking command (Block340). That is, the controller 44 may cause the current command of brake43 to be maintained, for example by way of feedback from the sensor 50.This may correspond with time T₃ in the example of FIG. 4. Thereafter,the controller 44 may calculate an amount of braking torque currentlybeing applied by brake 43 to machine 10, and a corresponding pressureinside of the oncoming clutch that would be required to generate anequivalent amount of torque (Block 345). The braking torque may becalculated in any manner known in the art. In the disclosed example, thebraking torque is calculated based on the braking command (e.g., thefrozen pressure) and known geometry of brake 43. Similarly, thecorresponding pressure inside the oncoming clutch may be calculated inany manner known in the art. In the disclosed example, the correspondingclutch pressure is calculated by multiplying the braking torque by acoefficient that is specific to known geometry of the oncoming clutch.

After completion of block 345, the controller 44 may then beginmodulating the pressures of the oncoming clutch and the force on thefriction material of brake 43 such that a near net zero torque handoffis achieved (Block 350). In other words, the controller 44 may beginreducing the command of the brake 43 at about the same time asincreasing the pressure of the oncoming clutch and by about the sameamounts, such that the torque reduction of brake 43 is about the same asthe torque increase of the oncoming clutch. In this way, the torque handoff may not be discernible to the operator.

In the disclosed example, the torque handoff between brake 43 and theoncoming clutch may be a linear handoff determined as a function oftransmission output speed. In particular, at TOS₂, the brake 43 may beapplying 100% of the retarding torque to machine 10, while at a slowerspeed TOS₃, the oncoming clutch should be applying 100% of the retardingtorque. And as the TOS slows from TOS₂ to TOS₃, the brake 43 maytransfer the load of retarding torque to the oncoming clutch in anamount proportional to the TOS progress from TOS₂ to TOS₃. For example,during a handoff beginning at 1500 rpm and ending at 300 rpm, when thecurrent TOS is 600 rpm (or about 75% of the way between 1500 rpm and 300rpm), the brake 43 will be carrying about 25% of the retarding torque ofmachine 10 and the oncoming clutch will be carrying about 75% of theretarding torque. In one embodiment, the controller 44 may first causethe oncoming clutch to pick up its share of the retarding torque beforecausing brake 43 to shed its corresponding share. It is contemplatedthat the torque handoff between brake 43 and the oncoming clutch couldbe non-linear, if desired.

During the torque handoff between brake 43 and the oncoming clutch, thecontroller 44 may continuously compare the current TOS to TOS₃ todetermine when the handoff can be completed (Block 355). Until TOS isabout equal to TOS₃, control may loop through blocks 350 and 355.Thereafter, the controller 44 may fully disengage brake 43 and fullyengage the oncoming clutch (Block 360). That is, the controller 44 mayfully release brake 43 and increase the pressure inside the clutch packof the oncoming clutch to a maximum pressure. Control may pass fromblock 360 to block 300.

The disclosed transmission system may improve shift quality during adesired high-speed directional shift. In particular, because thedisclosed transmission system may utilize braking during directionalshifting, the system may not be speed-limited by an amount of frictionmaterial in the directional clutches. This may allow for shifting at anyspeed, which may improve productivity and profitability in someapplications. In addition, because the shifting may be automaticallyimplemented, the torque handoff between braking and clutching may besmooth and rate-controlled for efficient and quick directional changes.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the transmission system ofthe present disclosure without departing from the scope of thedisclosure. Other embodiments will be apparent to those skilled in theart from consideration of the specification and practice of the controlsystem disclosed herein. For example, although the clutches 34 and 36and the brake 43 have been described as being pressurized to increase anamount of torque passing therethrough, it is contemplated thatdepressurizing these devices could alternatively result in the torqueincrease. It is also contemplated that the blocks of FIG. 3 could berearranged into a different order, if desired. For example, block 305may be completed at about the same time as or at some point after blocks310 and 315, if desired. It is intended that the specification andexamples be considered as exemplary only, with a true scope of thedisclosure being indicated by the following claims and theirequivalents.

What is claimed is:
 1. A transmission system for a machine having anengine and a traction device, the transmission system comprising: afirst directional clutch configured to transfer power from the engine tothe traction device in a first direction; a second directional clutchconfigured to transfer power from the engine to the traction device in asecond direction opposite the first direction; a brake configured toapply a retarding torque to the traction device; a sensor configured togenerate a first signal indicative of a speed of the transmissionsystem; an input device configured to generate a second signalindicative of an operator's desire to shift power transfer directionsfrom the engine to the traction device; and a controller incommunication with the first directional clutch, the second directionalclutch, the brake, the sensor, and the input device, the controllerbeing configured to: cause disengagement of the first directional clutchin response to the second signal; cause the brake to apply the retardingtorque to the traction device; determine an error value as a function ofthe second signal and a target transmission system speed; selectivelyincrease the retarding torque applied by the brake when the error valueincreases; and selectively transfer the retarding torque from the braketo the second directional clutch when a value of the second signal isless than a threshold value.
 2. The transmission system of claim 1,wherein the controller is configured to determine the error value as afunction of the second signal and a deceleration rate of the targettransmission system speed.
 3. The transmission system of claim 2,wherein the error accrues only when the deceleration rate deviates awayfrom a target deceleration rate.
 4. The transmission system of claim 2,wherein a rate of retarding torque change applied by the brake islimited by a previously recorded manual rate of deceleration.
 5. Thetransmission system of claim 1, wherein the controller is configured tocause the brake to apply the retarding torque to the traction device ata constantly increasing rate until a filtered derivative of the firstsignal indicates that the transmission system has started to decelerate.6. The transmission system of claim 5, wherein the controller isconfigured to determine the error value only after the first signalindicates that the transmission system has started to decelerate.
 7. Thetransmission system of claim 6, wherein the controller is furtherconfigured to selectively reduce the retarding torque applied by thebrake regardless of error after the first signal indicates the speed ofthe transmission system is less than the threshold value.
 8. Thetransmission system of claim 7, wherein the threshold value isassociated with a capacity of the second directional clutch to assumeretarding torque from the brake.
 9. A method of automatically shiftingtraveling directions of a machine having an engine, a traction device,and a transmission connecting the engine to the traction device, themethod comprising: transferring power from the engine through a firstdirectional clutch of the transmission to the traction device in a firstdirection; receiving input indicative of an operator's desire totransfer power from the engine to the traction device in a seconddirection; and disengaging the first directional clutch in response tothe input; causing a brake to apply a retarding torque to the tractiondevice; sensing a speed of the transmission; determining an error valueas a function of the speed and a target transmission speed; selectivelyincreasing the retarding torque applied by the brake when the errorvalue increases; and selectively transferring the retarding torque fromthe brake to a second directional clutch of the transmission when thespeed is less than a threshold value.
 10. The method of claim 9, whereindetermining the error value includes determining the error value as afunction of the speed of the transmission and a deceleration rate of thetarget transmission speed.
 11. The method of claim 10, wherein the erroraccrues only when the deceleration rate deviates away from a targetdeceleration rate.
 12. The method of claim 10, wherein a rate ofretarding torque change applied by the brake is limited by a previouslyrecorded manual rate of deceleration.
 13. The method of claim 9, whereincausing the brake to apply the retarding torque includes causing thebrake to apply the retarding torque at a constantly increasing rateuntil a filtered derivative of the speed of the transmission has startedto decelerate.
 14. The method of claim 13, wherein determining the errorvalue includes determining the error value only after the speed of thetransmission has started to decelerate.
 15. The method of claim 14,further including reducing the retarding torque applied by the brakeregardless of error after the speed of the transmission is less than thethreshold value.
 16. The method of claim 15, wherein the threshold valueis associated with a capacity of the second directional clutch to assumeretarding torque from the brake.
 17. A machine, comprising: an engine; atraction device; a transmission having a first directional clutch, asecond directional clutch, and a plurality of gear ratio clutches, thetransmission being configured to transfer power from the engine to thetraction device in first and second directions and throughout a range ofspeed-to-torque ratios; a brake configured to apply a retarding torqueto the traction device; an input device configured to generate a firstsignal indicative of an operator's desire to shift power transferdirections of the transmission throughout operation of the transmissionwithin the range of speed-to-torque ratios; a sensor configured togenerate a second signal indicative of a speed of the transmission; anda controller in communication with the transmission, the brake, theinput device, and the sensor, the controller being configured to: causedisengagement of the first directional clutch in response to the secondsignal; cause the brake to apply the retarding torque to the tractiondevice; determine an error value as a function of the second signal anda target transmission deceleration rate; selectively increase theretarding torque applied by the brake when the error value increases;and selectively transfer the retarding torque from the brake to thesecond directional clutch when a value of the second signal is less thana threshold value.
 18. The machine of claim 17, wherein the erroraccrues only when the second signal indicates a deceleration rate of thetransmission deviating away from the target transmission decelerationrate.
 19. The machine of claim 18, wherein: the controller is configuredto cause the brake to apply the retarding torque to the traction deviceat a constantly increasing rate until a filtered derivative of thesecond signal indicates that the transmission has started to decelerate;and the controller is configured to determine the error value only afterthe second signal indicates that the transmission has started todecelerate.
 20. The machine of claim 19, wherein: the controller isfurther configured to selectively reduce the retarding torque applied bythe brake regardless of error after the second signal indicates thespeed of the transmission is less than the threshold value; and thethreshold value is associated with a capacity of the second clutch toassume retarding torque from the brake.